This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-257280, filed Sep. 10, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a charge beam exposure apparatus, a charge beam exposure method, and an charge beam exposure mask for adjusting beams and exposing patterns according to patterns to be exposed.
Conventionally, a character projection exposure system (referred to hereafter as a CP exposure system) and a scanning exposure system are designed as exposure systems using an electron beam.
First, the following explains the CP exposure system using a schematic diagram in FIG. 1. As shown in FIG. 1, a region as large as several micrometers including a plurality of figures is formed as CP apertures 101a through 101e on a second formation aperture 101. The CP exposure system collectively exposes these CP apertures 101a through 101e. This exposure system is assumed to be promising as means for improving throughput. Specifically, an electron beam irradiated from an electron gun 11 is formed to a specified shaped through a first aperture 13. The shaped electron beam enters a second aperture 101. Specified CP apertures 101a through 10le are selected from the second aperture 101, forming an electronic image having a pattern composed of the selected CP apertures 101a through 101e. This electronic image is collectively exposed on a wafer 19.
When the CP exposure is used, a total electric current amount for beams varies with an opening area or an area for collective exposure. When a CP aperture which is 5 by 5 micrometers square is used for exposure, for example, a focus position fluctuates approximately up to 30 micrometers compared to an ordinary Variable Shaped Beam (VSB). Accordingly, the CP exposure requires the focus position optimization which is unnecessary for the VSB system.
For example, Jpn. Pat. Appln KOKAI Publication No. 2837515 discloses a beam adjustment method for such an electron beam exposure apparatus. FIG. 2 shows a structure of the second aperture used for the adjustment method described in this Jpn. Pat. Appln KOKAI Publication. A second aperture 201 is provided with a rectangular aperture 203 and an optical axis adjustment opening 204 in addition to a pattern exposure opening 202. In the conventional method, the optical axis adjustment opening 204 is used to adjust an optical axis.
However, this beam adjustment method has the following problems.
The conventional method uses an optical axis adjustment opening 204 to perform a beam adjustment for adjusting a beam focus and a beam deflection amount depending on a change in beam currents by varying an amount of beams that pass through rectangular apertures 203.
However, this beam adjustment method is available only when an unsimplified electro-optic system is used. FIG. 3A illustrates an apparatus structure for an unsimplified electro-optic system. FIG. 3B illustrates an apparatus structure for a simplified electro-optic system. These figures use an electron gun 161, a capacitor lens 162, a first aperture 163, a formation lens 164, a second aperture 165, a reduction lens 166, an objective lens 167, and a sample 168.
The simplified electro-optic system in FIG. 3B can be used when the scanning exposure system is used or the CP system is used for exposing all patterns. Namely, when the scanning exposure system is used or the CP system is used for exposing all patterns, it is unnecessary to form a first aperture image on a second aperture image. Accordingly, the formation lens 164 in FIG. 3A is unnecessary.
Here, an external edge of a rectangular aperture is referred to as an aperture edge. An edge of an electron beam is referred to as a beam edge. For decreasing a beam amount, a beam is irradiated to only part of the aperture edge. When the beam is irradiated so as to overlap the rectangular aperture, the beam edge is shaded by the rectangular aperture. When the beam is irradiated to only part of the aperture edge, the electron beam at the beam edge is also transferred onto the wafer 19. A beam at this beam edge is defocused on the wafer 19. This beam defocusing degrades the beam adjustment accuracy. Namely, the conventional beam adjustment is applicable only for the electro-optic system in FIG. 3A, in which the formation lens 164 is used for forming the first aperture image and the projection lens 166 and the objective lens 167 are used for forming the second aperture image on the sample surface. Apparently, the method in FIG. 3A is incapable of a beam adjustment according to variations in beam current amounts for the optical system in FIG. 3B.
The following describes a second problem with reference to FIGS. 4A and 4B. When the exposure apparatus in FIG. 1 is used for beam adjustment, a heavy metal dot 111 in FIG. 4A is provided on the wafer 19 or a stage (not shown) where the wafer 19 is placed. A CP-shaped electron beam 12 is applied to this heavy metal dot 111 through the use of surface scanning. The electron beam 12 is irradiated to shaded regions. An operation is performed to find a two-dimensional beam intensity distribution obtained by this surface scanning. Further, an operation is performed to find an ideal two-dimensional beam intensity distribution.
After the surface scanning, a beam adjustment is performed using this ideal intensity distribution as a template based on the actually obtained beam intensity distribution and the correlational method. Alternatively, it is possible to perform a beam adjustment and the like with respect to an edge position of the electron beam 12. FIG. 4B shows a two-dimensional intensity distribution of a reflected electron signal. The abscissa axis represents beam scanning positions. The ordinate axis represents reflected electron signal amounts.
However, the use of the above-mentioned signal processing method creates the following problem. Compared to the ordinary VSB, it takes a long time to find a two-dimensional beam intensity distribution using the surface scanning. Further, when the focus position is displaced for dozens of micrometers, not only the focus, but also a beam position or a beam rotation is affected. Consequently, the beam adjustment requires a lot of additional works.
To solve these problems, a method is designed to form a pattern having the same shape as the electron beam shape on the sample surface and to detect marks and adjust beams using this pattern. This method is examined with respect to an electron beam exposure apparatus for the scanning exposure system.
FIGS. 5A and 5B illustrate this beam adjustment method. As shown in FIG. 5A, the electron beam 12 is scanned in the direction of an arrow with reference to a CP alignment mark 121 formed on the sample surface. Then an operation is performed to find an intensity distribution for reflected electron signals before and after the electron beam 12 passes the CP alignment mark 121. The electron beam 12 is irradiated to a shaded region which is same as a pattern shape of the CP alignment mark 121. As shown in FIG. 5B, a resulting reflected electron signal becomes maximum when the CP alignment mark 121 completely coincides with the electron beam 12. Applying this beam adjustment method to beam adjustment, say, for CP exposure eliminates at least the need for the surface scanning and accompanying image processing and the like, making the beam adjustment simple.
However, when there are more than 100 types of CP apertures, for example, the beam adjustment for CP exposure would consume a vast amount of time in view of different numerical apertures or CP areas.
The following describes problems in the scanning exposure system.
FIG. 6A is a schematic diagram of the scanning exposure system. The scanning exposure system exposes the electron beam 12 irradiated from the electron gun 11 on the wafer 19 via the first aperture 13. This is same as the CP exposure system. The scanning exposure system provides a transfer mask 131 as shown in FIG. 6B instead of a second aperture 101. This transfer mask 131 differs from the second aperture 101 used for the CP exposure system. Regardless of availability of a repetitive pattern and the like, the transfer mask 131 has a pattern which is to be transferred to the wafer 19 and is exactly represented as exposure blocks 132a through 132f. 
A pattern formed on this transfer mask 131 is divided into exposure blocks 132a through 132f with a specified size. The electron beam 12 is vertically deflected on a pattern provided in each of exposure blocks 132a through 132f. At the same time, the transfer mask 131 is displaced parallel. Patterns in exposure blocks 132a through 132f are reduced and transferred onto the wafer 19.
However, this scanning exposure system creates the following problems. As shown in FIG. 6B, different pattern densities are used such as for exposure regions 132f-1 and 132f-2 even within the same exposure block 132f. In this case, different current densities are found in the electron beam 12 traveling from the transfer mask 131 to the wafer 19. Consequently, defocusing occurs in accordance with pattern densities in exposure regions 132f-1 and 132f-2. FIG. 6C shows transfer patterns for exposure regions 132f-1 and 132f-2. A dimensional difference occurs between transfer patterns 133-1 and 133-2 which correspond to the respective regions. In addition, there is a problem that different pattern densities cause misalignment in drawing positions for transfer patterns 133-1 and 133-2.
However, the transfer mask and the exposure method according to the prior art have not taken effective measures for avoiding these problems.
The beam adjustment method for the conventional electron beam exposure has problems as follows. For example, many labors are needed for optimizing the focus position, the beam position and the like. Alternatively, a long time is required for calculating the two-dimensional beam intensity distribution based on the sample surface.
It is an object of the present invention to provide a charge beam exposure apparatus capable of a precise, fast beam adjustment.
It is another object of the present invention to provide a charge beam exposure method capable of a precise, fast beam adjustment.
It is yet another object of the present invention to provide a charge beam exposure mask capable of a precise, fast beam adjustment.
According to a major aspect of the present invention, the present invention provides a charge beam exposure apparatus, comprising: a mask having a plurality of patterns with different pattern densities, a stage provided with a substrate to which patterns are transferred, and a controller. The controller has a drawing parameter table. This table stores optimal beam adjustment parameters using pattern densities as parameters for charge beams. Based on the drawing parameter table, the controller controls charge beams irradiated on the substrate. There is provided a charge beam exposure apparatus characterized in that a pattern transfer is performed by selecting an optimum beam adjustment parameter corresponding to the pattern density from the drawing parameter table for each of the above-mentioned patterns.
According to another aspect of the present invention, there is provided a charge beam exposure method for adjusting beams on a mask provided with a plurality of patterns having at least two pattern densities for each of these patterns and irradiates a charge beam to transfer the pattern on the sample surface. A pattern having different pattern densities is transferred to a sample surface to form a plurality of beam adjustment marks. Alternatively, these marks are formed with almost the same shape as the pattern which is already transferred to the sample surface. There is provided a charge beam exposure method characterized by having a first process, a second process, and a third process. The first process is used for irradiating a charge beam onto a plurality of beam adjustment marks, varying beam adjustment parameters for each pattern corresponding to the beam adjustment mark, detecting an electron reflected on the sample surface, and finding an optimum beam adjustment parameter. The second process is used for creating a drawing parameter table which stores optimum beam adjustment parameters using pattern densities as parameters. The third process is used for selecting an optimum beam adjustment parameter based on the drawing parameter table and for transferring a pattern onto the sample surface.
The present invention previously finds a beam adjustment parameter optimal for a pattern density. Further, the present invention creates a drawing parameter table based on this beam adjustment parameter and exposes a pattern. During exposure, an optimal beam adjustment parameter for the pattern density is selected from the drawing parameter table. It is possible to provide precision patterning by decreasing dimensional differences due to pattern density differences, drawing position fluctuations, and the like.
It is needless to find beam adjustment parameters for all pattern exposure openings. A beam adjustment is available by finding beam adjustment parameters only for representative points according to pattern densities. It is possible to provide an easy, fast beam adjustment.
Since a beam adjustment pattern is formed on a substrate, it is possible to create a beam adjustment mark by means of exposure using an ordinary Variable Shaped Beam. Further, just replacing a substrate provides easy maintenance when the beam adjustment mark is contaminated. Since the substrate is replaceable, the beam adjustment mark can be easily changed when a different pattern is used after replacement of a mask, for example.
According to another aspect of the present invention, the present invention provides a charge beam exposure mask which is provided for adjusting irradiation conditions of charge beams used for exposure. The charge beam exposure mask comprises a plurality of beam adjustment openings with different pattern densities and a plurality of pattern exposure openings used for pattern exposure.
Preferably, at least one of the beam adjustment openings is placed among a plurality of pattern exposure openings. Also preferably, a plurality of pattern exposure openings constitutes at least two pattern exposure opening groups comprising at least two pattern exposure openings as mentioned above. The beam adjustment opening is placed among a plurality of pattern exposure opening groups as mentioned above. Alternatively, the beam adjustment opening is placed nearer to a center of the mask than to a plurality of pattern exposure opening groups as mentioned above.
This mask structure shortens a distance for the mask to move during beam adjustment. Accordingly, the beam adjustment finishes in a short time and improves productivity for the charge beam exposure.
According to yet another aspect of the present invention, the present invention provides a charge beam exposure method which applies a beam adjustment to a mask provided with a plurality of patterns having at least two pattern densities for each of the corresponding patterns and irradiates a charge beam to transfer the pattern onto the sample surface. There is provided the charge beam exposure method which monitors a contamination situation of the mask based on the amount of electrons passing the mast pattern.
Accordingly, it is possible to determine proper timing to replace or clean the mask.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.